Three-phase induction motors have been widely recognized as workhorses of many industrial applications. Because of dramatic improvements in magnetic and thermal properties of permanent magnet materials, synchronous PM motors represent viable alternatives within this group, e.g. due to its high efficiency. From the control point of view permanent magnet synchronous motors are appropriate for applications with load-independent speeds or synchronous operation with high accuracy under defined speed.
Precise speed control of AC motors is typically accomplished with a frequency converter connected to a shaft speed or position feedback sensor. For controlling a PM synchronous motor ideally, the rotor angle has to be known by the control system. There are many known methods for identification the rotor angle, e.g. injecting AC signal or spatially distributed DC current pulses to the stator terminals and exploiting anisotropy in analyzing the signal response when determining the rotor orientation. Regardless of rotor structure the polarity detection of d-axis is normally based on stator saturation; current fed along the flux reduces the inductance more than current fed against the flux, making the polarity of d-axis observable. This, however, is not the case with all PM motors. In some cases, due to the motor design it may result that stator is not saturating with practical amount of current, which means that identifying the d-axis polarity and thus also the rotor exact angle cannot be done.